1. Field of the Invention
The present invention relates to a method of generating image data and an apparatus therefor which can prevent any influence of discontinuous change of gradation levels (hereinafter referred to as "tone jump") in recording of images having continuous gradation levels on a recording plane in the form of arrays of pixels having discrete gradation levels.
2. Description of the Prior Art
In an image recording apparatus such as a process scanner, gradation of images are recorded on a recording plane by arrays of pixels having discrete gradation levels. Therefore, an image whose gradation levels are continuously varied with respect to position, as typically shown in FIG. 1B, is approximately expressed on the recording plane as an image having a stepped gradation change as shown in FIG. 1A. Such a situation is not restricted to process scanners, but is common to apparatuses for performing image recording by employing digitized image data.
On the other hand, various improvements have been introduced into image recording apparatuses so that such a discontinuous change in gradation, i.e., a tone jump .DELTA.G in FIG. 1A is not visualized. Thus, such a tone jump .DELTA.G is hardly visualized in most cases of image recording. However, presence of such a tone jump .DELTA.G is relatively clearly visualized in the following case:
A first example is such a case wherein gradation levels are monotonically changed at a designated gradation change rate as in the case of recording of an inclined halftone image in a process scanner. In such a case, smoothness of gradation change is frequently blocked by the tone jump .DELTA.G.
A second example is an image such as that of human skin, gradation levels of which are slowly changed to easily draw attention of the person who sees the recorded image. Also in this case, presence of the tone jump .DELTA.G is easily recognized as turbulence in the recorded image.
A third example is the case of producing a recorded image on a large recording plane by enlarging an original image. In this case, pixels are increased in size by enlargement processing, whereby presence of the tone jump .DELTA.G is inevitably recognized when detailed observation is visually performed.
Various techniques have been considered in order to cope with such problems, and description is now made on typical examples together with disadvantages thereof.
According to a first conventional technique, data length of image data i.e., the bit number of digital signals expressing gradation levels is made sufficiently large to decrease the absolute value of the tone jump .DELTA.G. This method is most desirable in principle. In practice, however, the amount of data is increased with increases in data length while the processing time is considerably lengthened. Further, circuits for performing such a large amount of data processing must be prepared, whereby the apparatus is increased in size and cost.
In a second conventional technique, digitized image data are once converted into analog signals to be exposed to natural noise, such as circuit noise, applied to the analog signals. In this method, the analog signals exposed to the noise are again converted into digital signals to be employed for image recording. The natural noise or the like is so employed that pixels, causing the tone jump .DELTA.G, cannot be specified on observing the same. However, since the level of such natural noise is generally small, the bit number of the digital image data must be considerably large in order to employ this method. Thus, this method has a disadvantage similar to that of the aforementioned first conventional method.
In a third conventional technique, random digital noise having a certain degree of level is generated by a noise generator, and added to the digital image data. Dissimilarly to the first and second methods, a certain degree of effect can be expected in this method. However, this method has such a problem that modification of image data is performed independent of the position where the tone jump .DELTA.G is caused since noise irrelevant to the characters of the images to be recorded is added to the image data.
Thus, in these prior art examples, it has been difficult to effectively eliminate the influence of the tone jump .DELTA.G in recording of images whose gradation levels are continuously changed.